The state-of-the-art Li-ion battery technology is facing formidable challenges, because of the increasing demands for high capacity and large-current application scenarios [1]. For example, to supply power to electrical vehicles that last 300-500 miles with a single charge. In this context, scientists have switched their attention to lithium-sulfur battery because of its high theoretical energy density of up to 2600 W h kg−1 (corresponding to a specific capacity of 1675 mA h g−1 for sulfur cathode), five times higher than that of lithium-ion batteries based on conventional cathodes [2-7]. However, the commercialization of lithium-sulfur battery remains hampered by two major obstacles: (i) the intrinsic poor electronic conductivity of sulfur (5×10−30 S cm−1) and its discharging products (Li2S and Li2S2), making the overall utilization of sulfur electrode very low and limiting such batteries' rate performances, and (ii) the high solubility of the polysulfides formed upon repeated charge/discharge cycles; such polysulfides can shuttle from cathode to anode, where they deposit in the form of solid Li2S/Li2S2, leading to the continuous consumption of cathode material, and resulting in severe cycle life degradation [8-10].
One possible way to improve the conductivity of sulfur electrode and to alleviate the shuttle effect mentioned above would be to construct a composite [11, 12], and in view of this, a number of composites have been fabricated including carbon-sulfur composites [5, 13-16], polymer-sulfur composites [17-22], metal oxide-sulfur composites [23], etc. Among the many candidate matrixes, graphene, a 2D and one atom-thick carbon layer, has been tailored to host sulfur in battery applications [24-35], for its superior electrical conductivity, excellent mechanical flexibility, and high theoretical surface area. So far, most studies have adopted two methods to prepare graphene/sulfur (G/S) composites: i) thermal infusion strategy (using liquid-solid interaction in which graphene is soaked in melted sulfur) [26, 27, 32, 35], and ii) the solution-based synthesis (using the reaction of either sodium thiosulfate or sodium polysulfide with acid to precipitate sulfur in graphene suspension) [24, 25, 29, 31, 33, 34]. However, as a typical “2D” material, graphene itself cannot effectively confine sulfur (and polysulfides) inside the composite due to its intrinsic geometrical characteristics [33, 36].
Strong chemical bonding between graphene and sulfur (and its discharging products) is essential for improving the electrochemical performance of lithium-sulfur batteries [37-40]. And the proper engineering of interfacial chemistry between graphene and sulfur should revive G/S composite for future applications.